The present invention relates to an objective lens system used in a device that reads data from an optical disk, by emitting converging laser light onto a reflective layer of the optical disk.
An optical disk such as a compact disk or a magneto-optical disk is composed of a substrate made from a resin or glass material, having a reflective aluminum layer coated on the substrate. The reflective surface of the optical disk stores digital audio, video or data information in the form of `pits` and `bumps`. The laser beam is transmitted through the substrate and reflected by the reflective layer. The pits and bumps modulate the reflected laser beam such that the stored digital information can be recovered.
Japanese Patent Laid Open Publication No. SHO 61-56314 discloses a known objective lens system in which both light refracting surfaces are aspherical. In the optical system of the above-mentioned publication, the converging laser light is incident on the substrate, which acts as a parallel plate and as a result, produces spherical aberrations. In order to obtain a good convergence of the laser light, the spherical aberration and comatic aberration (coma) of the entire optical system (i.e., the objective lens system and the substrate of the optical disk), were corrected.
When the objective lens system of this optical system is positioned so that the optical axis is perpendicular to the substrate, the aberrations are reduced to a low level. However, when the optical axis of the objective lens system is not perpendicular to the substrate, the convergence of the laser light on the reflective layer deteriorates because of the inherent coma of the objective lens system, since the aberrations produced by only the objective lens system were not corrected. When the convergence of the laser beam deteriorates, the diameter of the laser beam which is incident on the reflecting layer becomes larger. This increases the likelihood that an error in writing or reading data from the optical disk, will occur.
The convergence of the laser beam is determined by the positional relationship between the optical disk, the objective lens system and the angle of incidence of the laser beam on the objective lens system. In the conventional optical system the optical axis of the objective lens system must be maintained perpendicular to the optical disk in order to prevent the deterioration of the convergence of the laser light. Therefore, a complex adjustment of the objective lens system is required in order to align the optical axis of the objective lens system with the normal to the optical disk surface.
Further, since the objective lens system and the optical disk are driven independently, if the objective lens system is inclined during a tracking operation or a focusing operation, coma of the laser beam incident on the reflecting layer becomes larger even if the objective lens system is correctly adjusted for the case that the optical disk is not being rotated.
Conventionally, if the relationship between the optical disk and the objective lens system is correct, the convergence of the laser beam does not deteriorate that much even if the angle of incidence of the laser beam is inclined with respect to the optical axis of the objective lens system. However, if the optical axis of the objective lens system is inclined with respect to a normal of the optical disk, the convergence of the laser beam will deteriorate.